In typical reproduction devices, such as copiers or printers, for example, information is reproduced on individual cut sheets of receiver material such as plain bond or transparencies. Receiver sheets, of the various types, are stored in stacks and respectively fed seriatim from such stacks when copies are to be reproduced thereon. The sheet feeder for the reproduction devices should be able to handle a wide range of sheet types and sizes reliably and without damage. Desirably, the sheets are accurately fed individually from the sheet stack, that is, without misfeeds or multi-feeds.
Reproduction device sheet feeders are typically of two types, vacuum feeders or friction feeders. However, of the two types, friction feeders are typically the least reliable, because sheet materials exhibit a wide variation in friction characteristics. Nevertheless, an exemplary vacuum sheet feeder is shown in a U.S. Pat. No. 5,344,133, issued Sep. 6, 1994, in the name of Jantsch et al. In such an apparatus, a stack of sheets is stored in a supply hopper. A sheet feed head assembly, including a plenum, a vacuum source in flow communication with the plenum, and a mechanism, such as a feed belt associated with the plenum, urges a sheet acquired by vacuum in a sheet feeding direction away from the sheet supply stack.
Typically, in most vacuum sheet feeders, the sheet supply stack is supported to maintain the topmost sheet at the feed head assembly. A first positive air supply then directs a flow of air at the sheet supply stack to levitate the top several sheets in the supply stack to an elevation enabling the topmost sheet to be acquired by vacuum from the sheet feed head assembly plenum. Additionally, a second positive air supply typically directs a flow of air at an acquired sheet to assure separation of any additional sheets adhering to such topmost sheet.
It is clear that the sheet stack should be maintained in operative relation with the sheet feed head assembly to assure desired feed from the stack. An exemplary control of a sheet stack is shown in a U.S. Pat. No. 5,823,527, issued Oct. 20, 1998, in the name of Burlew et al. In such an apparatus, a sheet feeder is disclosed having a platform for supporting a stack of sheets, a feed head assembly for feeding sheets seriatim from the top of a sheet supply stack on the platform, a mechanism for moving the platform relative to the feed head assembly, and device for controlling operation of the platform moving mechanism. The control device can determine a selected parameter in response to examination of sheet stack parameters, and consequently produce a signal corresponding thereto. The speed of the platform moving mechanism is then set based on the parameter signal.
In a typical vacuum sheet feeder, a portion of the stack is usually first lifted or “fluffed” and then sheets are fed off this fluffed group, singularly. At some point in time, the height of the top of the fluffed group is preferably low enough to allow for a paper level sensor to deactuate, and thus, signal a lift command to the motor. Generally, this occurs prior to feeding the last sheet of that fluffed group. If not, more sheets are lifted off the top of the unfluffed portion of the stack.
However, for certain types of receivers, it has been found for most notably heavyweight paper with poorly cut edges that a portion of the top of the stack is sometimes lifted, such that the level sensors remain actuated, even as the fluffed portion is being fed. Once this fluffed portion is fed, the next sheet will not be pre-separated from the rest of the stack, and consequently, the top of the remaining stack will be a greater distance below the vacuum plenum than is desired. This can lead to an undesirable increase in the probability of feed errors.
The embodiments described herein allow for more effectively controlling the level of a sheet stack.